creating new slice by appending to existing slice in golang

Question

I am writing a program to generate all possible permutations of a slice of unique elements. The code for the same can be found here

At one point I need to create a new slice from the original slice of numbers. This new slice has one less element than the original slice. I tried two different variants with append

nums is the original slice and remaining is the new slice I am trying to create. i is an int which can range from 0 to len(nums)-1.

Variant 1:

remaining := make([]int, 0)
remaining = append(remaining, nums[:i]...)
remaining = append(remaining, nums[i+1:]...)

Variant 2:

remaining := append(nums[:i], nums[i+1:]...)

While the program works fine with Variant 1 , it doesn't behave correctly with Variant 2. I want to understand what is the exact difference in these two Variants ?

You are using slice having same array backing, means `nums` also modified see [here](https://play.golang.org/p/cAwYaFJDxuC) — Eklavya, Aug 20 '20 at 08:15
I recommend you explore [slices-intro](https://blog.golang.org/slices-intro) — Eklavya, Aug 20 '20 at 08:21
neither `nums` nor `remaining` is ever mutated? What exactly is causing the modification? — Abhishek Jha, Aug 20 '20 at 08:24
See this answer: [Concatenate two slices in Go](https://stackoverflow.com/questions/16248241/concatenate-two-slices-in-go/40036950?r=SearchResults#40036950). — icza, Aug 20 '20 at 08:34
Do not use `append` to create a new slice, use it to append to a slice. To create a new slice, use a slice literal or `make()`. To copy data from an existing slice to the new slice, use `copy()`. — Adrian, Aug 20 '20 at 13:37

score 0 · Accepted Answer · answered Aug 20 '20 at 08:51

Here slice nums[:i] is created by slicing a bigger array nums. This leads to it having enough capacity to extend in place. So an operation like append(nums[:i], nums[i+1:]...) causes the elements in nums getting overridden by elements from nums[i+1:]. This mutates the original array and hence the behaviour.

As suggested by @icza the concept has been capture here.

To fix the Variant 2 we can use full slice expression like this

remaining := append(nums[0:i:i], nums[i+1:len(nums):len(nums)]...)

creating new slice by appending to existing slice in golang

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